The present invention relates to a control system and method for a hydraulic fluid pump whose output is controllable and which is incorporated in an automatic transmission; and more particularly relates to such a control system and method for such a pump which provide good and appropriate hydraulic fluid pressure for the various operating elements of said transmission in a wide range of engine operational conditions.
A typical automatic transmission for a vehicle comprises a gear transmission mechanism which can selectively provide any one of a plurality of forward speed stages (gearing ratios) between its power input shaft connected to the crankshaft of the vehicle engine via a torque converter and its power output shaft connected to the driven road wheels of the vehicle, so as to transmit rotational power therebetween. And such a gear transmission mechanism, which typically comprises a plurality of selectively engagable gear trains, is set in a determinate way to one or the other of said plurality of forward speed stages according to the current values of various operational parameters of the internal combustion engine and of the vehicle incorporating it, by engagement and disengagement in appropriate combinations of various friction engaging devices (hydraulic clutches and hydraulic brakes) incorporated in said transmission, under the control of a control system. In other words, this control system, on an ongoing basis according to the particular current values of said operational parameters, supplies actuating hydraulic fluid pressures to those appropriate ones of said friction engaging devices which are currently required to be engaged, while not supplying such actuating hydraulic fluid pressure to those appropriate ones of said friction engaging devices which are currently required not to be engaged. And the manner in which said plurality of forward speed stages is thus selected between according to the current values of said vehicle operational parameters is conventionally described by one ormroe shift diagrams.
Now, a torque converter for such an automatic transmission is typically filled with hydraulic fluid which circulates in the general circulation pattern of a smoke ring around a toroidal shape defined by a pump impeller, a turbine member, and a stator member, and accordingly a supply of pressurized hydraulic fluid is constantly required for said torque converter. Similarly, for being supplied as actuating hydraulic fluid pressures to those appropriate ones of said friction engaging devices which are currently reuired to be engaged, such a supply of pressurized hydraulic fluid is again required, typically a supply of so called line hydraulic fluid pressure which is selectively switched by one or more speed switching valves to be supplied to the friction engaging devices. This supply is typically provided by a hydraulic fluid pump; and, since this pressurized hydraulic fluid supply is required whether the vehicle is moving or not, and in all of the speed stages of the automatic transmission, it is usual to drive such a hydraulic fluid pump from the power input shaft of the automatic transmission as a whole, i.e. typically from the pump impeller member of the torque converter, which rotates along with the rotation of the crankshaft of the vehicle engine.
In the case of the simple type of hydraulic fluid pump which has no particular regulation means, its output, considered either in terms of fluid volume or pressure, increases along with increase in the rotational speed of the crankshaft of the engine. Since typically the maximum value of the pressure generated by the hydraulic fluid pump is regulated by means of a primary line pressure regulator valve of the spill type, this means that above a certain engine rotational speed the amount of hydraulic fluid vented by this line pressure regulator valve increases, which wastes engine power, accordingly resulting in vehicle operational inefficiency and lowered fuel economy.
In order to cope with this problem, various variable capacity type hydraulic fluid pumps have been proposed. See, for example: SAE Technical Paper Serial No. 790725 (Society of Automotive Engineers, June 1979); Jidoosha Kogaku Zensho (Automotive Engineering Papers) "Power Transmission Device", 15 Nov. 1980, Sankaido (pub.) pp. 273-238; and Nissan Technical Report No. 19 (December 1983), pp. 70-76; none of which it is intended hereby to admit as prior art to the present patent application except to the extent otherwise required by law.
In these proposals, there is proposed a variable capacity pump, the capacity of which is reduced either in response to increase of said pump output or in response to increased crankshaft rotational speed. Accordingly, the output of the pump is strived to be kept constant, and this means that wasted extra pump output is minimized, and accordingly power loss is avoided or reduced.